An electrostatic discharge, which can be referred to as an ESD, is a sudden flow of electric current from one electrically charged conductor to another conductor, sometimes through an insulator. Specifically, a large potential difference across the insulator generates a strong electric field, converting the material's atoms into ions that conduct a current. Electrostatic discharge is a serious problem in electronics, because integrated circuits are made from materials such as silicon, which can break down if exposed to high voltages or high currents. As such, an electrostatic discharge can physically damage or destroy integrated circuits along with associated electronics thereby rendering them essentially useless.
One type of electronic technology that can be susceptible to electrostatic discharge is capacitive touchpad technology, which is part of a broad category of capacitive touch-sensing technology enabling communication of user input to a computing device or other electronic device. A capacitive touchpad includes a sensing region that a user typically touches with a finger, a stylus, or some type of probe. The sensing region is commonly made operative by connecting its sensing electrodes to electronic circuitry, which are all susceptible to damage by electrostatic discharge events.
One traditional way to provide electrostatic discharge protection to a capacitive touchpad involves incorporating a continuous grounded conductive ring into the touchpad that encircles its sensor electrode region. Although this approach has been effective in protecting the sensor region from electrostatic discharge strikes, by virtue of the fact that the conductive ring is grounded, it can be problematic. For example, when a conductor such as moisture is present on the surface of the touchpad facesheet, the conductive ring introduces undesirable capacitive coupling to ground by sensor electrodes located near the conductive ring. As such, the moisture-induced coupling to ground causes the touchpad to operate inconsistently since sensor electrodes located near the conductive ring respond differently than electrodes located further away from the conductive ring, such as, in the center of the sensing region.
In order to reduce this moisture induced coupling to ground, the conductive ring is electrically (ohmically) disconnected from the system's ground and is left floating. Accordingly, any coupling is to the floating conductive ring rather than to a grounded conductive ring. This form of coupling reduces moisture related artifacts. However, it also reduces or eliminates the effectiveness of the conductive ring in protecting the sensor electrodes and their associated sensing circuitry from electrostatic discharge events.
Another traditional way to provide electrostatic discharge protection to an electronic system such as a touchpad involves incorporating diode shunts. Specifically, diodes may be used to protect low voltage signal lines. As such, two diodes are typically connected between the signal line and ground, one in each direction. If the voltage exceeds the switching voltage of the diode, the current is conducted through one of the two diodes. Unfortunately, diodes are often unable to shunt a sufficient amount of current and circuit damage may result.
The present invention may address one or more of the above issues.